diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
index b858b8d0f7..a0217adbf5 100644
--- a/src/backend/utils/mmgr/generation.c
+++ b/src/backend/utils/mmgr/generation.c
@@ -855,7 +855,12 @@ GenerationRealloc(void *pointer, Size size, int flags)
 	 *
 	 * XXX Perhaps we should annotate this condition with unlikely()?
 	 */
+#ifdef MEMORY_CONTEXT_CHECKING
+	/* With MEMORY_CONTEXT_CHECKING we need an extra byte for the sentinel */
+	if (oldsize > size)
+#else
 	if (oldsize >= size)
+#endif
 	{
 #ifdef MEMORY_CONTEXT_CHECKING
 		Size		oldrequest = chunk->requested_size;